1. Field of the Invention
The present invention relates generally to the field of treatment and prophylaxis of retinal degenerative diseases. More particularly, the present invention contemplates a method for preventing, reducing the risk of development of, or otherwise treating or ameliorating the symptoms of, age-related macular degeneration (AMD) or related retinal conditions in mammals and in particular humans. The present invention further provides therapeutic compositions enabling dose-dependent or dose-specific administration of agents useful in the treatment and prophylaxis of age-related macular degeneration or related retinal degenerative conditions.
2. Description of the Prior Art
Reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that that prior art forms part of the common general knowledge in any country.
Bibliographic details of references in the subject specification are also listed at the end of the specification.
Macular degeneration is a clinical term that is used to describe a family of diseases that are all characterized by a progressive loss of central vision associated with abnormalities of Bruch's membrane, the choroid, the neural retina and/or the retinal pigment epithelium. These disorders include very common conditions that affect older subjects—such as AMD as well as rarer, earlier-onset dystrophies that in some cases can be detected in the first decade of life. Other maculopathies include North Carolina macular dystrophy, Sorsby's fundus dystrophy, Stargardt's disease, pattern dystrophy, Best disease and Malattia leventinese.
AMD is the leading cause of permanent vision loss for individuals over age 65, currently affecting approximately 15 million Americans. AMD affects light-sensitive photoreceptor cells and pigmented epithelial cells in the macula, the center of the retina of the eye. While it may not cause total blindness, the disease destroys central vision, making reading, watching electronic monitor screens and driving impossible. It has no documented cure, has never demonstrated spontaneous remission and effective treatments are very limited.
The retina is a complicated network of nerve cells that changes light into nerve impulses that travel to the brain where they are interpreted as visual images. The central part of the retina, called the macula, is responsible for vision that is needed for reading and other detailed work. Damage to the macula results in poor vision. The most common disease process that affects the macula is AMD. In patients with AMD, retinal photoreceptor and pigment epithelial cells in the macula die over the course of several years. The cell death and gradual visual loss usually do not begin until age 60 or older, hence the name age-related macular degeneration.
There are two types of AMD: dry macular degeneration and wet macular degeneration. Dry macular degeneration, although more common, typically results in a less severe, more gradual loss of vision. Patients who are affected by dry AMD have gradual loss of central vision due to the death of photoreceptor cells and their close associates, retinal pigmented epithelial (RPE) cells, with deposition of a complex waxy amyloid mixture, termed ‘drusen’. Photoreceptors, the cells in the retina that actually ‘see’ light, are essential for vision. Macrophagic RPE cells are necessary for photoreceptor survival, function and renewal.
Patients with wet macular degeneration develop new blood vessels under the retina. As the photoreceptor and RPE cells slowly degenerate, there is a tendency for blood vessels to grow from their normal location in the choroid into an abnormal location beneath the retina. This abnormal new blood vessel growth is called choroidal neovascularization (CNV). The abnormal blood vessels leak and bleed, causing hemorrhage, swelling, scar tissue, and severe loss of central vision. Only 10% of patients with AMD have the wet type, but it is responsible for 90% of all blindness resulting from AMD.
The RPE cells in the eye act as macrophages, which phagocytize and recycle components of the membranous outer segments of photoreceptors. If the mitochondria within the RPE cells are damaged, the photoreceptor recycling is inhibited, with resultant accumulation of drusen. Drusen causes a lateral stretching of the RPE monolayer and physical displacement of the RPE from its immediate vascular supply, the choriocapillaris. This displacement creates a physical barrier that may impede normal metabolite and waste diffusion between the choriocapillaris and the retina.
Depending on the location, laser treatment can sometimes be given to destroy the abnormal blood vessels formed in wet AMD. Only 15% of the cases of wet AMD are eligible to have laser treatment because the blood vessels can not be located too close to the center part of the macula. The laser is a beam of light that is absorbed by the pigment of blood, drugs and RPE cells, which converts to heat energy that cauterizes the abnormal blood vessels. Frequently the neovascularization returns, since the stimulus has not been removed, resulting in severe loss of vision. In fact, most of the patients with AMD, who have very poor vision, have lost it due to sequelae of neovascularization. Current medical opinion states that there is no treatment available that permanently prevents the cell death or abnormal blood vessel growth that occurs in AMD.
To date, there are no known specific measures to prevent the occurrence of AMD. For patients already diagnosed with AMD in one or both eyes, current main treatments include light targeting (phototherapy) and/or a vitamin and mineral supplement, each of which is of debatable value. Phototherapy involves targeting light to the macular area containing the lesion of nascent defective blood vessels to inhibit or impair their function. One type of phototherapy is photodynamic therapy (PDT). In PDT, a photosensitive agent is administered into the vessels of a patient, then the agent is activated at the target site of the lesion of new vessels (the macula) by directing low energy light from a laser specifically to this area. The activated agent generates free radicals and other activated chemical species which destabilize and destroy the new vessels.
PDT has been reported to be of some benefit to patients having AMD. For example, one study, (Arch. Ophthalmol. 117:1329-1345, 1999) evaluated PDT in four hundred and two eyes from patients diagnosed with AMD in at least one eye. Treatment outcome was assessed by comparing the patient's ability to accurately read a conventional vision chart (one having about five letters per line) pre-treatment and post-treatment. At twelve months post-PDT, 61% of the eyes (246/402) lost fewer than 15 letters (that is, the patient lost less than about three lines on a standard visual chart), while 46% of the eyes (96/207) from patients undergoing treatment with a placebo lost fewer than 15 letters (p<0.001). At twenty-four months post-PDT, the visual acuity and contrast sensitivity was sustained in patients receiving PDT. A significantly greater percentage of these patients (58%) lost fewer than 15 letters, compared to patients undergoing treatment with a placebo (38%). However, only 16% of the patients receiving PDT had improved vision, compared to 7% of the patients receiving a placebo.
Another type of phototherapy is photocoagulation therapy. In photocoagulation therapy, high energy light from a laser is directed specifically to the target site of the new vessels. The heat generated from the high energy laser coagulates the fluid in and around the new vessels. Laser photocoagulation is not a form of PDT; it is a separate treatment approach. It uses lateral transfer of heat, applied with a cautery-like method, to coagulate fluid within and surrounding the vessel, while PDT uses an activated photosensitive agent to generate active chemicals which damage or destroy the new vessels containing the agent.
While either PDT or laser photocoagulation therapy is separately used to treat patients with AMD, neither is without drawbacks. A problem with PDT is that its effects are transient; patients receiving PDT must be retreated about every three months. Furthermore, the patients require at least five retreatments within the first two years merely to stabilize their condition, and before any therapeutic effect occurs. These cumulative treatments damage the retina, further reducing the patient's visual acuity.
One drawback of laser photocoagulation is that it is non-selective, and does not target only the new blood vessels. It must therefore be administered so that only the lesions are targeted, and the unaffected surrounding tissues are undamaged. However, in about half of the patients with AMD, the new vessels are located in the subfoveal area, which is difficult or impossible to target with laser coagulation without damaging the sensory retina. Another drawback is that photocoagulation treatment is not permanent and recurrence rates for new vessel production are high, reaching 39-76%, usually within the first two years. However, repeated treatments can actually induce the growth of new vessels and membranes (subretinal neovascular membranes and recurrent choroidal neovascularizations) at the site of the treatment. Repeated treatments may also irreversibly damage unaffected areas of the retina, including the neurosensory retinal and RPE. Thus, the treatment itself may result in the patient having further reduced vision over a period of time. Specifically, some patients undergoing photocoagulation therapy develop scotoma, which is an area of depressed vision within the visual field, surrounded by an area of less depressed or of normal vision.
There is a need, therefore, to develop alternative methods to treat AMD or related conditions.